Chromium-tungsten alloys



United States Patent signor to Commonwealth of Australia, Canberra, Australian Capital Territory, Commonwealth of Australia No Drawing. Filed May 2, 1960, fier. No. 25,861 Claims priority, application Austraiia Oct. 22, 1956 3 Claims. (Cl. 75il76) This invention relates to chromium-tungsten alloys and has for its object the provision of new alloys having good structural properties at high temperatures and particularly suitable for such applications as high performance gas turbine rotor blading.

Alloys according to the invention are characterised in that they consist essentially of l to 12% tungsten, 0.1 to of a further alloying component selected from the group consisting of titanium, niobium, molybdenum, silicon and mixtures thereof, and the balance chromium and incidental impurities, the chromium content amounting to not less than 85 by weight of the total alloy composition, and the impurities including gaseous impurities, iron, aluminum and other metals not exceeding a total of 0.5%.

The lower limit of 85 of the chromium content has been found necessary to ensure that the alloys meet the following requirements for gas turbine rotor blading:

(a) An acceptably low density. The tensile stress on rotor blading is directly proportional to the density of the mium, can be readily machined with normal machine tools. Since the alloys are required to be finished to the close tolerances of turbine rotor blading, this easy machine-ability is an essential feature.

(d) Formability. No high-strength alloy which consists preponderantly of chromium with the addition of heavy metals can be formed by the accepted metalworking operations unless it contains at least chromium. However, the ability to be worked by extrusion, forging, rolling and like procedures is frequently required in alloys for turbine rotor blading and this ability is in general possessed by the alloys of the present invention.

It has been ascertained that satisfactory properties for the purpose of this invention are possessed by alloys containing 1 to 12% tungsten and 0.1 to 5% of one or more of the additional elements titanium, niobium, molybdenum and silicon, with the balance not less than 85% chromium. There may also be impurities in the alloys, totalling not more than 0.5%, this total including gaseous impurities such as nitrogen and oxygen and non-gaseous impurities such as iron and aluminium. The amount of tungsten in the alloys is preferably in the range 2 to 6% and the amounts of titanium and niobium respectively are preferably in the range 0.1 to 2%.

At temperatures of 950 to 1000 C. alloys within the range specified have shown minimum creep rates under stress which compare favourably with the creep rates under similar or less severe conditions of a known high strength heat resistant alloy at present in commercial use as shown in Table I which now follows:

Table 1 Stress Test Minimum Tertiary Alloy Condition (in tons/ Temp. creep rate creep sq. in.) (in C (in. lin./hr.) after (hrS.)

Commercial high strength heat resistant alloy: 20% Solution treated 1,150 6 950 -150X10- 20-21 chromium, 16% cobalt, 3% titanium; 2% aluminum, 0. and aged 700 C. 12 950 1,600X10" 2% balance nickel o t e 1 000 ssxro- 312 Chromium, tungsten. niobium {fi 33 5555 15355 1 1 6 000 5 1 Cast 12 950 8.I 10- 1 284 Chromium, 10% tungsten, 0.5% niobiurmgo 13 1, iigfiglo Chromium, 5% tungsten, 0.2% titanium Cast 6 1,000 15 10- 1 24(2) Chromium, 5% tungsten, 0.5% Silicon I g 1' ,gi Chromium, 5% tungsten, 0.5% niobium, 2% silicon. {g l fii g i 838 igzag: Chromium, 2% tungsten, 2% molybdenum, 0.3% 1110- {Cost 6 1,000 12X10 320 bium, 0.2% titanium, 0.5% silicon. Extruded 6 1, 000 63X10- 1 210 1 Test discontinued. No tertiary creep.

alloy used, so that the accepted density for currently used turbine alloys is substantially less than 9 grams/cc. To meet this requirement, alloys of chromium with heavy metals must contain not less than 85% chromium.

(b) Good resistance to high temperature oxidation. The naturally high oxidation-resistance of chromium is seriously impaired if it is alloyed with a total of more than 15% of poorly oxidation-resistant heavy metals.

(0) Acceptable hardness and machineability. Alloys of chromium with heavy metals containing less than 85% chromium come into the category of hard metals, noted for their resistance to Wear and to machining operations.

present invention, containing not less than 85% chro- The commercial alloy tested had a minimum creep rate of approximately l0 10 /hr. under a stress of 12 tons/ sq. in. at a temperature of 815 C. so that the alloys of the invention provide an advance of up to C. on the existing alloy. On a weight comparison, up to 10% tungsten the chromium-tungsten alloys of this invention are lighter than the comparison alloy while above 10% tungsten they are heavier.

While the primary strengthening addition in the alloys is tungsten, the high temperature creep resistance is much enhanced by the further additions of titanium, niobium and molybdenum, and the first two of these also greatly improve the ductility and general handling characteristics of the alloys. The strengthening effect of silicon is milder,

but it causes a marked increase in oxidation resistance, as shown in Table II which now follows:

A high strength, heat-resistant alloy composition coming within the scope of the present invention consists essentially of 2 to 6% tungsten, 1 to molybdenum, 0.1 to 2% of a further alloying component selected from the group consisting of titanium, niobium, silicon and mixtures thereof, and the balance chromium and incidental impurities, the chromium content amounting to not less than 85% by weight of the total alloy composition, and the impurities including gaseous impurities, iron, aluminum and other metals not exceeding a total of 0.5%

Alloys according to the invention can be hot-worked at high temperatures while a proportion of the alloys can be cold worked at lower temperatures.

For hot-working, high speed extrusion is employed, preferably using uncooled steel dies and a molten glass lubricant. The alloy is heated for extrusion in a protective atmosphere of hydrogen or an inert gas, such as argon or helium, to minimize surface contamination by atmospheric gases, particularly nitrogen, This heating method is also used for hammer forging, which is conducted in the same temperature range. It is possible to produce a considerable range of shapes by hot-working. In particular, the extrusion method can be used to make aerofoil sections, with or without thickened root sections, suitable for the production of turbine blades.

Thus it has been found possible to extrude the alloy containing 5% tungsten and 0.5 niobium and other alloys in the specified range at temperatures above 1200 C. starting with a 1 /2 to 2" diameter arc-cast ingot and finishing with a A" to /8" diameter rod. Small arecast ingots of alloys within the specified range and containing up to 10% tungsten can be hammer-forged at temperatures above 1200 C. to a useful extent using particular handling techniques and one blow per heat. The main feature of the handling techniques is the use of tongs and holders made of solid molybdenum. These usually remain in contact with the chromium alloy ingots during the whole of the period of heating prior to forging, thus minimizing the heat loss which would other Wise occur in transferring the ingots from the furnace to the hammer face.

The strength at high temperature of alloys according to the invention is such that cold-working must usually be conducted at temperatures well above room-temperature, in the range 500 1300 C.

Heat treatment, if employed, consists of an ageing treatment from 5 to 50 hours at the intended service temperature, or up to 200 C. above this temperature. The normal range for this treatment is 900 to 1200 C. This may or may not be preceded by a high-temperature solution treatment in the range 1200 to 1600 C.

The alloys according to the invention may be used in the cast condition, or in the hot-worked or cold-worked conditions, with or without heat-treatment. The materials are intended primarily for the rotor and stator blading of gas turbine engines, and in other applications requiring high load-carrying ability at temperatures of 900 C. and higher.

The alloys can be shaped by normal machining methods, using high-speed steel or carbide tools, or grinding wheels with a copious supply of cutting lubricant.

This application contains subject matter in common with, and is a continuation-inpart of, my patent application Serial No. 850,523, filed November 3, 1959, which latter application in turn was a continuation-in-part of my parent patent application Serial No. 690,891, filed October 18, 1957; both of these patent applications have since become abandoned.

I claim:

1. A high strength heat resistant alloy consisting essentially of l to 12% tungsten, 0.1 to 5% of at least one further alloying component selected from the group consisting of titanium, niobium, molybdenum, silicon and mixtures thereof, and the balance chromium and incidental impurities, the chromium content amounting to not less than by weight of the total alloy composition, and the impurities including gaseous impurities, iron, aluminium and other metals not exceeding a total of 0.5

2. A high strength heat resistant alloy as defined in claim 1. containing 2 to 6% tungsten, and 0.1 to 2% of said further alloying component.

3. A high strength heat resistant alloy as defined in claim 1, containing 2 to 6% tungsten, 1 to 5% molybdenum and 0.1 to 2.0% of an additional member of said further alloying constituent group. 

1. A HIGH STRENGTH HEAT RESISTANT ALLOY CONSISTING ESSENTIALLY OF 1 TO 12% TUNGSTEN, 0.1 TO 5% OF AT LEAST ONE FURTHER ALLOYING COMPONENT SELECTED FROM THE GROUP CONSISTING OF TITANIUM, NIOBIUM. MOLYBDENUM, SILICON AND MIXTURES THEREOF, AND THE BALANCE CHROMIUM AND INCIDENTAL IMPURITIES, THE CHROMIUM CONTENT AMOUNTING TO NOT LESS THAN 85% BY WEIGHT OF THE TOTAL ALLOY COMPOSITION, AND THE IMPURITIES INCLUDING GASEOUS IMPURITIES, IRON, ALUMINIUM AND OTHER METALS NOT EXCEEDING A TOTAL OF 0.5%. 